The invention is concerned with a method of production of iron aluminide materials as well as iron aluminide base alloys which occur as an end product of a method of that kind.
From the patent application WO 90/10722 it is known that certain iron aluminide base alloys are suitable as the material for the execution of industrial constructions, in particular for constructions which must exhibit at high temperature (up to 650.degree. C.) and in an aggressive ambient (for example, H.sub.2 S+H.sub.2 +H.sub.2 O) a good resistance to corrosion as well as good mechanical strength. Such alloys present themselves, for example, as a cheap substitute for nickel-base alloys or high-alloy steels. Iron aluminides which consist mainly of Fe.sub.3 Al are distinguished by an orderly crystalline structure with DO.sub.3 -symmetry: the one half of the lattice sites which form a cubical lattice are occupied by Fe atoms; the other half of the lattice sites which lie spatially centred with respect to the cubes of the first lattice, exhibit a checkerboard-like arrangement of Fe and Al atoms. The alloy on the iron aluminide base is an orderly intermetallic alloy. In what follows it is called the Fe.sub.3 Al base alloy. The proportion of the aluminium in this alloy with a DO.sub.3 -structure exhibits a value in the range between 18 and 35% by atomic weight. Besides the DO.sub.3 -structure there is partially present in the Fe.sub.3 Al base alloy a B2 structure (or CsCl-structure) or a disorderly spatially centred alpha-structure.
In the case of known Fe.sub.3 Al base alloys with which are admixed up to 10% by atomic weight of chromium and in smaller amounts molybdenum, niobium, zirconium, yttrium, vanadium, carbon and/or boron, no low-melting-point eutectics are formed. Fe.sub.3 Al base alloys exhibit a protective layer of aluminium oxide covering the surface. However, iron aluminides and many of the Fe.sub.3 Al base alloys have a very poor ductility at room temperature. Only if the great brittleness of these materials can be overcome can they be employed as raw materials.
Ductility can as a rule be improved if by means of alloying additives the grain of the structure is made finer. From one publication (S. A. David et al (1989), Welding Research Sup., page 372), a Fe.sub.3 Al base alloy comprising 18-35% by atomic weight of Al, 3-15% by atomic weight of Cr, 0.2-0.5% by atomic weight of at least one of B and C, 0-8% by atomic weight of at least one of Mo, Nb, Zr, Y and V, and the remainder consisting of iron is known in which an increase in the ductility at room temperature has been achieved by means of the addition of titanium diboride (TiB.sub.2). In the case of welding experiments (by electron beam, arc welding), however, a hot crack formation was observed. Experiments with secondary ion mass spectrometry yielded that at the face of the crack boron and titanium occurred enriched. This discovery led to the following opinion: The titanium diboride goes into solution in the melt; it has no influence upon the formation of grain. Titanium and boron are not incorporated into the crystalline structure of the grains, therefore these constituents are to be found finally after the solidification of the Fe.sub.3 Al base alloy on the interfaces of the grains. Through the influence of heat during welding the force locking between adjacent grains becomes severely reduced because of the titanium diboride (because of local lowering of the melting point at the grain boundaries), so that a heat crack formation can arise. Consequently it is advisable in spite of improvement in ductility to waive the addition of titanium diborides or substances which lead to similar phenomena.